Atomic resolution storage (ARS) is performed by using a beam (i.e., a focused stream of electrons) to quickly heat and change the structure of a small section of matter (i.e., the media), possibly just a few atoms across. The size of the small section impacted by the beam is referred to as the spot size, which tends to decrease as focus improves and increase as the beam becomes defocused.
In some instances, the focused stream of electrons is provided by an emitter that is part of an array of focused electron beam sources, and the section of media is one of many such sections on the surface of the storage medium. The storage medium and the array of focused electron beam sources are usually moveable relative to each other to allow one focused electron beam source to access a plurality of sections on the storage medium.
Each focused electron beam source of the array typically includes at least a field emission tip (e.g., a cathode) and a gate that is positively biased and extracts electrons from the tip. Each tip and gate pair forms a tiny electron gun, which may include some additional electrodes such as focusing lenses and/or secondary gates. To obtain a small spot size, a focusing column with additional electron focusing lenses is typically introduced between the electron gun and the media which functions as an anode. In some instances, the field emission tip may be a Spindt tip. Spindt tips are tiny conical electron emitters, and are sometimes used in field emission displays (FEDs). Similar in principle to a CRT, a FED uses a beam of electrons to excite phosphors which then emit visible light.
Field emission tips, however, tend to be very noisy, particularly when emitting into a low vacuum (10−7-10−5 torr). This is due at least in part to the large influence that changes in geometry and/or material properties of the tip have on the emission current. Emission current (IE) is a measure of the amount of charge emitted by the field emission tip as a function of time. As such, it is difficult to get an emitter to produce a steady (i.e., relatively noiseless) emission current.
The difficulty in getting an emitter to produce a steady emission current is particularly significant in regard to ARS. For example, changing the structure of a section of the storage medium, as is frequently done in ARS, involves a balance between imparting enough energy to change the structure, and avoiding imparting so much energy that the storage medium surface is harmed, such as by ablation. Moreover, smaller spot sizes are desirable as smaller spot sizes permit higher density storage. Examples of ARS methods and apparatus which may be adapted to use the systems and methods disclosed herein can be found in U.S. Pat. No. 6,728,127, herein incorporated by reference in its entirety.
Attempts have been made to decrease or limit field emission noise (i.e., uncontrolled fluctuations of IE). Such attempts include the use of backing/ballast resistors, tip FET control, and gate/extractor control. Unfortunately, each of these methods tends to have a negative affect on spot size (i.e., electron beam focus). The spot size variations can negatively impact both reading and writing of data. As such, whether heretofore recognized or not, there is a need for better systems and methods for controlling electron beam emission, particularly in ARS systems.